A general method of producing a niobium capacitor from a niobium powder is as follows. First, a molded article of a niobium powder is produced with a niobium lead wire inserted therein. Next, by heating the molded article, a porous niobium sintered body is obtained wherein the niobium powders to each other and the lead wire and the surrounding niobium powder are sintered and electrically integrated. Furthermore, voltage is applied to the niobium sintered body wherein the lead wire side functions as an anode to anodize (also referred to as “form”) the sintered body and form a dielectric material film of niobium oxide on the surface thereof (including the inside surface of the pores). Then a cathode material such as manganese dioxide is charged into the space of pores of the niobium sintered body in a shape of a three-dimensional net. Furthermore, after laminating an electrically conductive paste on the surface of the niobium formed body, the niobium formed body is fixed on a lead frame and the whole is sealed with resin to obtain a capacitor.
A defect of a niobium capacitor is to have a large amount of leakage current flowing through the dielectric material film when a voltage is applied. This arises from the nature of niobium ready to take in the oxygen in the air.
When sintering the niobium incorporating oxygen, a niobium dioxide crystal is produced which is electrically conductive. Anodizing niobium generally produces an amorphous niobium oxide film on the surface of niobium, while anodizing niobium including a niobium oxide crystal produces an amorphous niobium oxide film containing crystalline niobium oxide. That is, the dielectric film of amorphous niobium oxide includes lots of conductors in the form of microcrystals. As a result, the leakage current of a capacitor increases and lowers the reliability.
The methods have been studied to reduce the leakage current of a niobium capacitor, and a technology has been developed to nitride the niobium powder and niobium sintered body. However, those methods have not necessarily reached to a level demanded by the market.